The present invention generally relates to the transport and storage of semiconductor wafers within a tool bay including various tools in a semiconductor wafer fabrication site, and in particular to a buffer, delivery and stocker system for wafer carriers.
Typically, in a semiconductor wafer fab each wafer is processed in several different tools for performing various wafer fabrication functions. The tools are arranged in so-called tool bays each comprising in general on the order of 15 to 20 process tools. Each tool bay is typically on the order of about eighty feet long. In order to enhance the throughput of wafers in the fab and/or to fabricate different wafers with different structures, there are usually multiple tool bays provided in the wafer fab.
The wafers are kept and transported as batches between the tools in carriers each being capable of accommodating about 25 wafers. Such carriers are mini-environments and are tightly sealed and are only opened to be loaded or unloaded. Thus it is ensured that particle fluxes onto semiconductor wafers in the carriers are reduced during storage and transport of the wafers through the semiconductor fabrication process. The effect of isolating the wafers is enhanced by the fact that the immediate environment (i.e. the surrounding air or gas) of the wafers is essentially stationary in the carriers and particles from outside do not arrive at the wafers.
For transport of the carriers within a wafer fab there is a first automated transport system provided for transferring the carriers between different processing tool bays (interbay delivery systems), and a second automated transport system provided for transferring the carriers around within each particular bay (intrabay delivery systems). Such interbay systems that are used for the transport of two carriers are well known in industry.
Within the tool bays there may be different types of tools, namely some processing tools with high throughput which are capable of performing their particular wafer process at a relatively high rate and other processing tools with a much lower throughput rate, e.g. metrology tools, which in general monitor or test a single wafer from within a pod of wafers. The different requirements as to providing wafer to (feed) the tools are to be matched in a tool bay in order to keep the respective high throughput tool from sitting idle. Hence, it is known to include a local tool buffer adjacent the tool port of high throughput and metrology tools, so that carriers may be stored locally adjacent such tools and quickly transferred to these tools without having to constantly retrieve a carrier from a remotely located stocker. Such stockers may be provided in each tool bay in order to buffer wafer carriers within the tool bay between subsequent processing steps. Each stocker typically has a plurality of shelves.
However, the above conventional transport systems have several disadvantages: firstly, there are numerous movements of the intrabay transport system necessary to retrieve wafer carriers from intrabay stockers or from the intrabay/interbay transport interface and to feed tools in the tool bay. Secondly, since the intrabay transport system has to serve several tools within said tool bay there may occur the situation that two different high throughput tools wait for new wafer carriers to process. In this case the intrabay transport system has to fetch a first wafer carrier from the intrabay/interbay transport interface or from a tool within the tool bay and to deliver it to one of the two waiting tools. Only then the intrabay transport system can retrieve the second wafer carrier for the other of the two waiting tools. So, the second of the two waiting tools stays undesirably idle for a certain time. Thirdly, the complex movements of the intrabay transport system necessitates a performance of the transport algorithm that corresponds directly to the number of loadports and transport vehicles of the intrabay transport system.
The present invention seeks to provide an improved transport system for wafer carriers that overcomes the above disadvantages of prior art systems.